Conventional two-channel sound systems exhibit shortcomings so as to allow enhanced performance in stereophonic and/or multi-lingual modes. Improved performance is desirable not only for audio with video applications, but also for audio-only applications. A universal compatible multi-channel sound system applicable to sound and television broadcasting (e.g., satellite or terrestrial TV broadcasting), digital audio broadcasting (terrestrial and satellite), as well as other non-broadcasting media appear to be attractive to the manufacturer, producer and consumer. Examples of such other non-broadcasting media include:
______________________________________ CATV Cable TV Distribution CDAD Cable Digital Audio Distribution ENG Electronic News Gathering (including Satellite News Gathering) IPC Interpersonal Communications (videoconferencing, video-phone, etc.) ISM Interactive Storage Media (optical disks, etc.) NDB Network Database Services (via ATM, etc.) DSM Digital Storage Media (digital VTR, etc.) EC Electronic Cinema HTT Home Television Theatre ISDN Integrated Service Digital Network. ______________________________________
The standardized ISO/MPEG-Audio Multichannel system is an audio subband coding system which can be used to transfer high quality digital multi-channel and/or multi-lingual audio information on limited capacity channels. One of the basic features of such system is the backwards compatibility to ISO 11172-3 IS coded mono, stereo or dual channel audio programs, and is designed for use in different applications.
In stereophonic applications, the use of an additional center loudspeaker channel C and two left and right (rear) surround loudspeaker channels Ls and Rs is recommended to augment the front left and right loudspeaker channels L and R. This format is referred to as "3/2-stereo" (3 front/2 surround loudspeaker channels), and requires the transmission of five appropriately formatted sound signals.
For video with sound applications (e.g. High Definition Television, or HDTV), the three front loudspeaker channels provide sufficient directional stability and clarity of picture related frontal images, according to common practice in cinema. The main benefit is a "stable center", which is provided at most listener locations, and is important for dialogue. Also, for audio-only applications, the 3/2-stereo format has been found to improve the two-channel stereo effect. The addition of one pair of surround loudspeaker channels, e.g., to the rear of the listener, provides improved realism of sound ambience. Basically, the transmission of the five audio signals of a 3/2 sound system requires five transmission channels. In the context of bit-rate reduced signals, these channels are not necessarily totally independent. In order for two of the transmitted signals to provide a stereo service on their own, the source sound signals are generally combined in a linear matrix prior to encoding. These combined signals (and their transmission channels) are identified by notations T1, T2, T3, T4 and T5.
T1-T5 generally designate the five audio transmission channels in accordance with the MPEG audio standard, e.g., ISO/IEC 13818-3. Briefly, T1 corresponds to the mixed audio signal actually transmitted in the standard left stereo channel Lo. T2 corresponds to the mixed audio signal actually transmitted in the standard right stereo channel Ro. T1 is mixed from signals L, C and Ls. T2 is mixed from R, C and Rs T3 corresponds to a pure L signal. T4 corresponds to a pure R signal. T5 corresponds to a pure Ls signal. For compatibility reasons, an audio decoder compatible with the ISO 11172-3 IS standard properly decodes the basic stereo information, consisting of basic left and right channels. The signals in these channels constitute an appropriate downmix of the audio information in all channels. The appropriate downmix equations are given by the following expressions (T1 and T2 correspond to Lo and Ro): EQU Lo=L+x*C+y*Ls (1) EQU Ro=R+x*C+y*Rs (2)
In the above expressions "x" and "y" represent factors (e.g., 0.707 and 0.5 as will be seen) which indicate shared portions of pure source signals L, R, C, Rs and Ls.
A parameter TC Allocation contains information concerning the transmission channel allocation in a subband group, as indicated by the following table. In accordance with the MPEG audio standard, the audio spectrum is divided into 32 subbands.
______________________________________ TC Allocation T3 T4 T5 ______________________________________ 0 L R C 1 L R LS 2 L R RS 3 L C LS 4 L C RS 5 L LS RS 6 R C LS 7 R C RS 8 R LS RS 9 C LS RS 10 L R -- 11 LS RS -- 12 L -- -- 13 R -- -- 14 C -- -- 15 -- -- -- ______________________________________
The TC Allocation parameter designations 0-15 indicate various combinations of the five basic source signals, i.e., transmission channel allocation for all subbands. Further details are described in MPEG document "ISO 11172-3 Compatible Low Bit Rate Multi-Channel Audio Coding and Conventional Stereo Coding at Lower Sampling Frequencies", ISO/IEC JTC1/SC29/WG11 N0403, MPEG93/479, 11. June 1993.